The field of the present invention is temperature control systems and, more particularly, systems and methods for effecting temperature control of high temperature superconducting thin film filter subsystems.
Recently, substantial attention has been devoted to the development of high temperature superconducting radio frequency (RF) filters for use in, for example, cellular telecommunications systems. However, such filters are extremely temperature sensitive. By their very nature, high temperature superconducting (HTSC) materials are temperature dependent. At temperatures above their "transition temperatures," the materials behave like an insulator, and at temperatures below the transition temperature, the materials become superconducting.
Further, when a HTSC film is fabricated into a RF filter, temperature fluctuations stemming from kinetic inductance of the filter may have a substantial effect upon the operation of the filter and, in particular, upon the center-frequency of the filter. Similarly, fluctuations in temperature may have a substantial impact upon certain non-linear behavior characteristics of HTSC thin film filters. While the non-linear behavior characteristics of a HTSC thin film filter may have a relatively mild effect upon filter operation at temperatures below the transition temperature, the same cannot be said for the kinetic inductance effect. Further, as the temperature of operation of a HTSC thin film filter approaches, for example, the transition temperature of the filter, relatively minor fluctuations in the operating temperature can have very significant effects upon filter operation. Stated somewhat differently, as HTSC thin film filter systems are operated closer and closer to their respective transition temperatures, more and more care must be taken to control the temperature of the operating environment. Thus, it will be appreciated that HTSC thin film filter systems must be maintained at stable operating temperatures if proper operation of the systems is to be maintained. This is particularly so where HTSC filters are to be operated at or near their respective transition temperatures.
Those skilled in the art also will appreciate that increased temperature stability generally is required when more "narrow-band" filters are utilized within a HTSC filter system. The reason for this is that relatively small changes in operating temperatures (e.g., +/-1.degree. K.) may have a substantial impact upon the range of filter operation, particularly if a filter is operated at or near its transition temperature. Indeed, such changes in operating temperature may cause the center frequency of a HTSC filter to vary by as much as 100 MHz.
Now, because maximum advantage may be obtained through the use of HTSC thin film filters when the filters are operated in a narrow-band mode at approximately the transition temperature, those skilled in the art will appreciate that it is highly desirable, if not essential, to maintain very precise control of the operating temperatures of HTSC thin film filter systems.
Those skilled in the art also will appreciate that, when multiple HTSC filters are disposed, for example, within the dewar of a cryocooler, and the cryocooler is mounted, for example, on a telecommunications tower, substantial temperature control issues may arise. Simply put, a tower-mounted cryocooler will need to provide more lift (i.e., more "cold") on a hot afternoon than would be required on a cold night. Further, as the ambient temperature of the environment within which a HTSC filter system is mounted varies, temperature gradients will result between the system cold source (i.e., the cold finger of the system cryocooler) and the cold stage or location where the HTSC filters are located.
Traditionally, the above-described problems have been solved by (1) operating a HTSC filter at temperatures well below the transition temperature of the filter; (2) controlling the temperature of a HTSC filter by adding heat to a so-called cold plate with some thermal conduction to a cooler; (3) over-designing a HTSC filter system so that required temperature specifications may be met even in the presence of substantial fluctuations in, for example, ambient temperature; (4) making a cold plate or HTSC filter mount very thick so as to reduce temperature gradients across the mount; and/or (5) providing for in-situ tuning of a HTSC filter system.
Those skilled in the art will appreciate that each of the above-described options represents only a partial solution to the HTSC filter temperature control issue, and that each option has inherent disadvantages. For example, option (1) represents a tradeoff between filter temperature stability and cooler size. In short, the use of a larger cooler may provide lower "cold-finger" temperatures and increased lift, but may result in higher power consumption and system heat generation. Option (2) represents a similar tradeoff, but in that instance, the issue that must be addressed is the addition of a new heat load to the system. Finally, those skilled in the art will appreciate that options (3) and (5), while potentially effective, are subject not only to economic limits, but also to performance limits.
In view of the foregoing, it is believed that those of ordinary skill in the art would find an improved temperature control system for use with HTSC thin film filter systems to be quite useful.